This project has as its goal the study of the in vivo molecular dynamics of muscular contraction. The principal approach used is the investigation of the intensity fluctuation spectra of laser light scattered by muscles, at rest and during contraction, and by in vitro preparation of muscle organelles and contractile proteins. So far we have established that the current cyclic cross-bridge model of muscular contraction is not sufficient to explain the "structural motion" as just the cycling of the cross-bridges. Entire myofibrillar sarcomeres move erratically and independently as much as 200 angstrom units in a msec. and change their sarcomere length and polarizability. These results are strong evidence for fluctuations in the force developed in individual myofibrillar sarcomeres; fluctuations that may arise from fluctuations in cross-bridge number. BIBLIOGRAPHIC REFERENCES: Fraser, A., Eisenberg, E. Kielly, W.W., and Carlson, F.D. The interaction of heavy meromyosin and subfragment-1 with actin: physical measurements in the presence and absence of ATP. Biochemistry 14, 2207-2214 (1975). Carlson, F.D. and Fraser, A. Dynamics of F-actin and F-actin Complexes. J. Mol. Biol. 89, 273-281 (1975).